There are a multitude of options open to the modern rock star wishing to announce to the world that they have embarked on a new album. You can give interviews, allow webcams into your studio, offer a free download as a taster. Or you can appear on Andrew Marr's Sunday morning politics show in a feathered headdress, playing the autoharp over an incessant, off-key sample of the Four Lads' 1953 hit Istanbul (Not Constantinople), while a nonplussed Gordon Brown looks on. Thus did PJ Harvey seek to remind the world that she is not like other singer-songwriters, useful clarification for anyone who felt her offering to sodomise an errant male with a strap-on dildo (on 2009's A Woman A Man Walked By) suggested an artist cravenly bidding for the Katie Melua demographic.

Somehow, the recent news that Harvey had frequently received career advice from the late Captain Beefheart didn't come as that much of a surprise. It says something that Let England Shake, the album she chose to announce on the Andrew Marr Show last April – an opaque exploration of Englishness delivered in a high, keening voice, that contains not one, not two, but three harrowing songs that explicitly reference the 1915 Gallipoli campaign and a further handful that seem more generally informed by the carnage of the first world war – represents one of the more approachable albums in her oeuvre. If it's not as straightforward as the slick FM rock found on the Mercury prize-winning Stories from the City, Stories from the Sea (Captain Beefheart was apparently not a fan of that one), at least its 40 minutes passes without anyone getting anything shoved up their bum.

In fact, listeners used to Harvey in full-on, bug-eyed Dorset avenger mode might be slightly taken aback by Let England Shake. Not because of the preponderance of gorgeous tunes – if, in the past, Harvey has been guilty of making records one admires for their bloody-mindedness rather than enjoys for their songs, she's also proved herself capable of turning on the melodic charm at will. It's more a matter of tone. The music sounds muted, misty and ambiguous, which seems to fit with Harvey's vision of England: "The damp grey filthiness of ages, fog rolling down behind the mountains and on the graveyards and dead sea captains," she sings on The Last Living Rose.

Scrupulously avoiding the usual cliches that arise with self-consciously English music – Kinksy music-hall observations, eerie pagan folkisms, or shades of Vaughan Williams – the central sound is guitars, wreathed in echo that makes them seem as if they're playing somewhere in the middle distance. Around them are scattered muzzy electric piano, smears of brass, off-kilter samples and musical quotations: a reference to Eddie Cochran's Summertime Blues somehow works its way into The Words That Maketh Murder, while an incessant trumpet reveille sounds during The Glorious Land, out of tune and time with the rest of the song. Somewhere along the way, the Four Lads have vanished – instead, their song's incongruously perky melody is played on a xylophone – but on Written on the Forehead, she performs a similar trick with an even more unlikely source – reggae singer Niney the Observer's Blood and Fire, a deceptively cheery paean to imminent apocalypse. Its weird juxtaposition of subject matter and mood infect the whole song, which is possessed both of a beautiful melody and a lyric about people trying to escape a rioting city and drowning in sewage.

Meanwhile, Harvey's voice certainly has its dramatic moments, as when it rockets into boy-soprano territory during On Battleship Hill, or unexpectedly takes on a carefully enunciated mock-aristocratic mien. But frequently what it most obviously evokes is a rather cool ambivalence. When she debuted her new high register on White Chalk, it sounded tremulous and spooked: here it's almost blank-eyed as she details The Words That Maketh Murder's battlefield carnage: soldiers falling "like lumps of meat", trees hung with severed limbs. It's a curious idea, but it's a masterstroke. Rock songwriters don't write much about the first world war, but, perhaps understandably, when they do, they have a tendency to lay it on a bit thick: you end up with songs like the Zombies' The Butcher's Tale, so ripe it sounds more like the work of a fromagier. Harvey clearly understands that the horror doesn't really need embellishing: her way sounds infinitely more shocking and affecting than all the machine-gun sound effects in the world.

You're left with a richly inventive album that's unlike anything else in Harvey's back catalogue. That, she told Marr last year, is the point: "My biggest fear would be to replicate something I've done before." Let England Shake sounds suspiciously like the work of a woman at her creative peak. Where she goes from here is, as ever, anyone's guess.